Some of the holes have closed, but the remaining gaps are mystifying officials.

"The field is is like, there is nothing, no trees or anything and you see everywhere some holes, some deeper than others, like 30 or 40 holes everywhere on the field. You can see this is not normal. You can see this is a problem on this land," said city spokesperson François Moisan.

Some of the affected fields used to be sandpits, but experts are running tests with surface radar instruments to analyze the soil.

Evacuation is optional, but strongly recommended, Moisan said.

"The city is very prudent and we ask people to leave their house because there is some danger. We don't know what kind of danger, we don't know if it's a real threat to their property but we prefer to take no chances and ask them to leave."

A map of the affected area in Quebec City's Charlesbourg neighbourhood. ((CBC/Radio-Canada))